1. Field of the Invention
The present invention relates to an active material for a battery that is useful as a negative-electrode active material of a lithium battery, for example, a battery that contains the active material for a battery, and a method for the production of the active material for a battery.
2. Description of Related Art
Lithium batteries have been put to practical use in many fields, such as information-related devices and communication devices, because of their high electromotive force and high energy density. In addition, rapid development of electrical vehicles and hybrid vehicles is also desired in the field of automobiles from the perspective of environmental protection and resource consumption, and lithium batteries are considered as power sources for such vehicles. A lithium battery usually has a positive-electrode active material layer that contains a positive-electrode active material, a negative-electrode active material layer that contains a negative-electrode active material, and an electrolyte layer that is formed between the positive-electrode active material layer and the negative-electrode active material layer.
A carbon material (such as graphite) is used as a negative-electrode active material of lithium batteries. On the other hand, an active material with higher thermal stability is desired to improve safety. Japanese Patent Application Publication No. 2008-123787 (JP 2008-123787 A) discloses a non-aqueous electrolyte battery in which lithium titanate (LTO) is used as a negative-electrode active material. Because LTO is an oxide, it has high thermal stability and is advantageous in terms of safety.
However, because LTO has an Li intercalation-deintercalation potential (oxidation-reduction potential), as measured with respect to metal Li (as measured versus metal Li), of about 1.5 V, which is higher than that of a carbon material (about 0.3 V), the resulting battery has a lower battery voltage. The battery voltage can be defined as the difference between the Li intercalation-deintercalation potential of the positive-electrode active material and the Li intercalation-deintercalation potential of the negative-electrode active material, for example. Thus, as the Li intercalation-deintercalation potential of the negative-electrode active material is higher, the resulting battery has a lower battery voltage as long as the same positive-electrode active material is used.
In “Electronically Driven Structural Distortions in Lithium Intercalates of the n=2 Ruddlesden-Popper-Type Host Y2Ti2O5S2: Synthesis, Structure, and Properties of LixY2Ti2O5S2 (0<x<2),” Geoffrey Hyett et al., Journal of the American Chemical Society, 126, 1980-1991 (2004), evaluation of physical properties of LixY2Ti2O5S2 is disclosed. In this literature, however, the physical properties of LixY2Ti2O5S2 are simply evaluated and no evaluation was conducted on battery properties. In addition, in “Electronically Driven Structural Distortions in Lithium Intercalates of the n=2 Ruddlesden-Popper-Type Host Y2Ti2O5S2: Synthesis, Structure, and Properties of LixY2Ti2O5S2 (0<x<2),” Geoffrey Hyett et al., Journal of the American Chemical Society, 126, 1980-1991 (2004), Li is forcibly introduced into Y2Ti2O5S2 to synthesize LixY2Ti2O5S2. However, there is no statement that Li can be deintercalated, and there is no statement that suggests that LixY2Ti2O5S2 can serve as an active material accordingly.
To obtain a high performance battery, an active material for a battery with high thermal stability and low potential is necessary. Based on previous studies, the present inventors have found that an active material for a battery that contains a Y2Ti2O5S2 crystalline phase can be one of active materials that satisfy the above requirements (PCT/JP2011/053502).